Relation of the decay of free radicals in irradiated polyethylene to the molecular motion of the polymer and the configurations of the free radicals

Decay reactions of the free radicals produced in irradiated polyethylene (high-density and low-density materials) were examined in connection with the molecular motion of the matrix polymer. Three temperature regions, in which the free radicals decay very rapidly, at around 120, 200, and 250°K, were designated T_A, T_L, and T_B, respectively. The decay of the free radicals at these temperatures had activation energies in high-density polyethylene of 0.4 kcal/mole for T_A, 9.4 kcal/mole for T_L, and 18.4 kcal/mole for T_B. In low-density polyethylene these quantities were 0.7 kcal/mole for T_A, 23.1 kcal/mole for T_L, and 24.8 kcal/mole for T_B. Comparison of time constants for the decay reactions and for molecular motion of the matrix polymer indicate that the decay in T_A and T_B is closely related to molecular motion in the amorphous regions of the polymer. The decay of the free radicals at T_L in high-density polyethylene is due to molecular motion associated with local mode relaxation at lamellar surfaces, while that of low-density polyethylene is due to local mode relaxation in the completely amorphous region. Steric configurations of the free radicals which decay in the respective temperature regions were also investigated.     

Ta2O5 thin-films deposited by off-axis and on-axis pulsed laser deposition techniques

The crystalline properties of Ta₂O₅ thin films deposited by an off-axis aperture-installation-type pulsed laser deposition technique are investigated and the results are compared with the results for films deposited by the conventional on-axis technique. No significant difference in the substrate temperature dependence is seen between films deposited on-axis or off-axis at a pulse repetition frequency of 30 Hz. When the repetition frequency is lowered, the degree of c-axis orientation is increased in both films. This tendency is more pronounced in the off-axis films. Therefore it is found that the off-axis aperture-installation-type pulsed laser deposition technique is effective not only for decreasing the density of droplets, but also for obtaining more highly c-axis-oriented crystalline films.     

Preparation of monodisperse,reactive hydrogel microspheres and their amphoterization

Monodisperse, reactive hydrogel microspheres were prepared by precipitation polymerization ofp-nitrophenyl acrylate (NPA) with acrylamide, methacrylic acid, and methylenebisacrylamide in ethanol. The size of microspheres was controlled by the monomer ratio. Some fraction of reactive ester decomposed during the polymerization. The reactive hydrogel microspheres were converted to amphoteric ones by the reaction of NPA units with diamine. The isoelectric point of the amphoteric microspheres was around 4.0, but it was different from the pH at which the microspheres have the minimum size or the most shrunken state. This was attributed to the uneven distribution of induced amine groups.     

Ionic products in ground polymer. IV. Anions in poly(vinylidene fluoride)

When poly(vinylidene fluoride) (PVDF) was ground with tetracyanoethylene (TCNE) powder in a vibration glass ball mill in vacuum in the dark at 77 K, the TCNE anion radical (TCNE-) was detected in the mixture by ESR spectroscopy. The TCNE.- is formed by the abstraction of electrons by TCNE from the anion that is produced by a heterogeneous bond scission of carbon-carbon bonds in the polymer main chain. The assignment of TCNE.- was carried out by the ESR spectral simulation on the basis of an anisotropic effective hyperfine tensor that includes a forbidden transition term. At least 85% of the bond scission of the PVDF main chain is induced by a heterogeneous process to produce ionic products. The yield of these mechanions seems to increase with the polarity of carbon-carbon bond owing to a substituent group bonded to the carbon in the polymer main chain.     

Flow injection analysis for traces of zinc with immobilized carbonic anhydrase

A small column packed with immobilized bovine carbonic anhydrase is used for determination of traces of zinc in aqueous solution, based on the measurement of recovered esterase activity of the metal-free apoenzyme after taking up zinc from the sample solution. Conditions for the removal of zinc from the immobilized enzyme and for activity measurement are established. A linear calibration graph is obtained between 1 × 10^?8 and 4 × 10^?7 g of zinc. Then the method is successfully applied to the determination of zinc in tea, sediment and spring water.     

Base-mediated cyclization reaction of 2-alkynylphenylphosphine oxides: synthesis and photophysical properties of benzo[b]phosphole oxides

The base-mediated intramolecular cyclization reaction of 2-alkynylphenylphosphine oxides affords benzo[ b]phosphole oxides, which show intense blue-green fluorescence. Benzo[ b]phospholes are also prepared by the reduction of benzo[ b]phosphole oxides.     

The pH effect on the naphthoquinone-photosensitized oxidation of 5-methylcytosine

The pH effect on the one-electron photooxidation of 5-methyl-2'-deoxycytidine (d(m)C) by sensitization with 2-methyl-1,4-naphthoquinone (NQ) was investigated. Photoirradiation of an aqueous solution containing d(m)C and NQ under slightly acidic conditions of pH 5.0 efficiently produced 5-formyl-2'-deoxycytidine, whereas similar NQ-photosensitized oxidation of d(m)C proceeded to a lesser extent under more acidic or basic conditions. Fluorescence-quenching experiments revealed that the less-efficient photooxidation at pH values below 4.5 is attributed to the decreased rate of one-electron oxidation of d(m)C owing to protonation at the N(3)-position. The NQ-photosensitized oxidation of an N(4)-dimethyl-substituted d(m)C derivative under various pH conditions also suggests that the pH change in the range of 5.0 to 8.0 may be responsible for a reversible deprotonation-protonation equilibrium at the N(4)-exocyclic amino group of the d(m)C radical cation. In accord with the photochemical reactivity of monomeric d(m)C, the 5-methylcytosine residue in oligodeoxynucleotides was oxidized efficiently by photoexcited NQ-tethered oligodeoxynucleotides under slightly acidic conditions to form an alkali-labile 5-formylcytosine residue.     

One-electron photooxidation and site-selective strand cleavage at 5-methylcytosine in DNA by sensitization with 2-methyl-1,4-naphthoquinone-tethered oligonucleotides

Photosensitized one-electron oxidation was applied to discriminate a specific base site of 5-methylcytosine (mC) generated in DNA possessing a partial sequence of naturally occurring p53 gene, using a sensitizing 2-methyl-1,4-naphthoquinone (NQ) chromophore tethered to an interior of oligodeoxynucleotide (ODN) strands. Photoirradiation and subsequent hot piperidine treatment of the duplex consisting of mC-containing DNA and NQ-tethered complementary ODN led to oxidative strand cleavage selectively at the mC site, when the NQ chromophore was arranged so as to be in close contact with the target mC. The target mC is most likely to be one-electron oxidized into the radical cation intermediate by the sensitization of NQ. The resulting mC radical cation may undergo rapid deprotonation and subsequent addition of molecular oxygen, thereby leading to its degradation followed by strand cleavage at the target mC site. In contrast to mC-containing ODN, ODN analogs with replacement of normal cytosine, thymine, adenine, or guanine at the mC site underwent less amount of such an oxidative strand cleavage at the target base site, presumably due to occurrence of charge transfer and charge recombination processes between the base radical cation and the NQ radical anion. Furthermore, well designed incorporation of the NQ chromophore into an interior of ODN could suppress a competitive strand cleavage at consecutive guanines, which occurred as a result of positive charge transfer. Thus, photosensitization by an NQ-tethered ODN led to one-electron oxidative strand cleavage exclusively at the target mC site, providing a convenient method of discriminating mC in naturally occurring DNA such as human p53 gene as a positive band on a sequencing gel.